This invention relates to the processing of alpha-2 titanium aluminide alloy articles fabricated by casting or ingot metallurgy to improve the microstructure of such articles.
Titanium alloy parts are ideally suited for advanced aerospace systems because of their excellent general corrosion resistance and their unique high specific strength (strength-to-density ratio) at room temperature and at moderately elevated temperatures. Despite these attractive features, the use of titanium alloys in engines and airframes is often limited by cost due, at least in part, to the difficulty associated with forging and machining titanium.
Recent developments in advanced hypersonic aircraft and propulsion systems require high temperature, low density materials which allow higher strength to weight ratio performance at higher temperatures. As a result, titanium aluminide alloys are now being targeted for many such applications. Titanium aluminide alloys based on the ordered alpha-2 Ti.sub.3 Al and orthorhombic Ti.sub.2 AlNb phases are currently considered as replacements for the much heavier nickel base alloys such as Inconel 718. These alloys have high specific stiffness, temperature strength and oxidation resistance. However, these alloys lack low temperature ductility, which makes handling, assembly and processing difficult.
Microstructure refinement is one of the most efficient methods to ductilize such alloys. Thermomechanical processing (TMP) methods, such as hot work followed by heat treatment, have been found to be effective in refining the microstructure, thereby increasing the room temperature ductility. Thermomechanical processing is not always an option, especially in net-shape technologies, such as casting or powder metallurgy.
It is widely known that the microstructure of the so-called "ordinary" titanium alloys, i.e., near-alpha, alpha-beta and near-beta alloys, can be refined by temporary alloying with hydrogen. Although hydrogen is beneficial as a transient alloying element for improving the hot workability and superplasticity of titanium and its alloys, pure titanium and many titanium alloys are embrittled at room temperature by the presence therein of only very small quantities of hydrogen.
Eylon et al, U.S. Pat. 4,820,360, issued Apr. 11, 1989, disclose that the fatigue resistance of cast titanium alloys can be increased by temporary alloying with hydrogen. Eylon et al, U.S. Pat. No. 4,872,927, issued Oct. 10, 1989, disclose that the fatigue resistance and superplastic deformation characteristics of wrought titanium alloys can be increased by temporary alloying with hydrogen.
In the area of the ordered titanium alloys, Eylon et al, U.S. Pat. No. 5,098,484, issued Mar. 24, 1992, disclose that the consolidation temperature of prealloyed alpha-2 titanium aluminide powder can be reduced by hydrogenating the powder, filling a die or mold with the powder, consolidating the powder and then dehydrogenating the resulting article.
In the case of cast and ingot metallurgy alpha-2 articles, modification of the microstructure by temporary alloying with hydrogen is severely limited by the relatively low rate of diffusion of hydrogen into the alloy and the limited solubility of hydrogen in the alloy. The optimum temperature for hydrogenation of the alpha-2 titanium aluminide alloys, in terms of maximizing the hydrogen content, varies with the alloy, but is generally about 40% below the beta-transus temperature of the alloy, in degrees C. However, at this temperature the rate of diffusion of hydrogen into the alloy is so slow that hydrogenation time becomes impractically long. The rate of diffusion can be increased by increasing the temperature, but the solubility level decreases with increasing temperature, thereby limiting the amount of hydrogen in solution to such low levels that do not allow microstructure refinement.
Accordingly, it is an object of the present invention to provide an improved process for refining the microstructure of cast and ingot metallurgy alpha-2 and orthorhombic titanium aluminide alloy articles.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art after reading the detailed description of the invention as well as the appended claims.